Hi Hamish,

First, as a general rule, I encourage clients to keep modifications to a minimum. I have plenty of "war stories" to share with them about clients who wanted so many changes to their base code that it became impractical to re-apply all those mods against future releases, thus they find it difficult to take advantage of upgrades the software vendor offers.

Then, once we do commit to changes, I comment the script at the top of the page with a "change log" that summarizes edits followed by individual comments in the code.  If I add a block of code, I do something that indicates "gbdg - new block added below" and "gbdg - new block added above".  Or if I add a database column to the selected fields on a SQL query, I add a comment ' gbdg- added XXX to query below.

This way when a new release is to be applied, I search for my (company initials) "gbdg" to find any pages that have been edited.  And pray that the client has followed suit...

I have also been known to retain a .xls worksheet listing all the changes and I and the client may have made.

And I personally like http://www.compareandmerge.com - www.compareandmerge.com for comparing old/new scripts.

You guys have pretty much covered it. We comment each change within a module with a "NetProfits - brief description of the change" comment or put start and end comments around larger blocks of code changes. We also keep a document per client that lists all modules that were changed with a description of the changes. We use Beyond Compare to update changes to new modules.